[BUG] set `dir = git_dir` will not make toggleterm try and derive the git repo directory

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

according to the documentation i setting the `dir` option to `git_dir` in neovim commandline like` ToggleTerm dir=git_dir`, but the directory in toggleterm does not change as expected

### Expected Behavior

this seems to be caused by vim.fn.isdirectory not recognizing git_dir. i find the function that toggleterm.nvim used to get or create terminal
``` lua
function M.get_or_create_term(num, dir, direction, name)
local term = M.get(num)
if term then return term, false end
if dir and fn.isdirectory(fn.expand(dir)) == 0 then dir = nil end
return Terminal:new({ id = num, dir = dir, direction = direction, display_name = name }), true
end
```
and find it depend `vim.fn.isdirectory()` to recognize directory, but `git_dir` will be not recognized.

Edited:
I added an additional if condition in my local `toggleterm.nvim`, which made `ToggleTerm dir=git_dir` work properly. I'm not sure if this will affect other behaviors.
``` lua
function M.get_or_create_term(num, dir, direction, name)
local term = M.get(num)
if term then return term, false end
-- HACK: `dir ~= "git_dir"` is a hack to make git_dir work
if dir and fn.isdirectory(fn.expand(dir)) == 0 and dir ~= "git_dir" then dir = nil end
return Terminal:new({ id = num, dir = dir, direction = direction, display_name = name }), true
end
```

### Steps To Reproduce

i set toggleterm.nvim in my configuration like this
``` lua
{
"akinsho/toggleterm.nvim",
version = "*",
event = "VeryLazy",
cmd = { "ToggleTerm", "TermExec" },
...
opts = {
on_create = function(term)
print(term.dir)
end,
autochdir = false,
direction = "horizontal",
shell = vim.fn.has("win32") == 1 and "pwsh" or vim.o.shell,
float_opts = {
border = "rounded",
},
},
}
```

### Environment

```Markdown
- OS: Windows 11 23H2
- neovim version: NVIM v0.10.0
- Shell: PowerShell 7.4.2
```

### Anything else?

_No response_